Catalytic reforming, using Pt based reforming catalyst, is one of the most important refinery processes in the world. Most refineries have a catalytic reformer, which converts naphtha fractions into high octane reformate.
Reformers come in many types and sizes--from 2000 BPD fixed bed units to moving or swing bed units processing more than 50,000 BPD. Reformers are available with fixed bed reactors, swing bed reactors, or moving bed reactors. Many new units are moving bed reactors, available from UOP, Inc, Des Plaines, Ill.
Reformers generally use mono-metallic catalysts (Pt on a support such as alumina) or bi-metallic catalyst (Pt-Re on a support). Other combinations of Pt and other metals are known. All reforming catalyst are believed to contain a halide, almost invariably chlorine. The presence of chlorine is beneficial for the reforming process, and may be essential for successful regeneration of Pt catalyst, as the Cl helps keep the Pt dispersed as small crystals on the catalyst.
While all reformers are believed to have some chloride compounds in the reformate, the problem is most serious when a continuous reformer is used, and especially so when the catalyst is near the end of its useful life.
Some refiners add chlorine compounds continuously to their units to maintain a high chloride level on the catalyst. In continuous or moving bed reformers the catalyst is chlorided after coke burn but before return to the top of the reforming reactor. More chlorine is added now, as opposed to 10 or 20 years ago, both as a prophylactic measure to allow the units to be pushed harder, and the belief that catalyst regeneration is more successful with more Cl on catalyst.
Cl in the reformate causes problems in downstream units. The main chlorine compounds in reformate are believed to be HCl, NH4Cl and FeCl3. Some refiners may use other halides, such as Fl or I, but Cl is the halide of choice, so hereafter chlorine and its reaction or degradation products will be referred to rather than halogens in general.
Chlorine compounds in reformate cause several problems. Some regions have a pH specification on gasoline, which can not be met if large amounts of HCl are present in the reformate. Chlorides can seriously affect downstream processing units, such as a Sulfolane aromatics extraction unit, if the reformate is so treated.
Chlorides can cause very immediate problems in the reformer. If the reformer is relatively dry, as most are, the chlorides form salts which plug up the reformer fractionators. If water is added to wash the salts out then HCl is formed, which causes serious corrosion problems. As an example, one of our refineries had a problem with chloride salt buildup in product fractionators. Every three months or so the fractionator efficiency declined so that it was necessary to water wash the column. About 1 wt % water was added to the tower to wash out salts. This cleaned the column, but would also form some HCl, which can attack some steels, especially with water present.
The problem has gotten worse in the last decade, going from nuisance to a major problem. The conventional methods of handling chlorine in reformate will be briefly reviewed. These are grouped arbitrarily below and reviewed in detail hereafter.
1. Water washing,
2. Solid adsorbent treating of reformate,
3. Chemical treatments.